In polymer type cells the individual cells are typically constructed with the cathode and anode materials being laminated to opposite sides of a thin solid polymer electrolyte separator membrane, with respective electrically conductive current collectors being placed adjacent to or within the anode and cathode. The anode, cathode, current collectors and separator together with electrolyte comprise the individual cell assembly which is typically placed into a metallized plastic laminate which is sealed under heat and pressure to form a completed sealed cell, with the respective current collectors remaining electrically externally accessible. The respective layers of the electrodes and electrolyte separator are very thin, generally on the order of 5-6 mils for the anode, 9-12 mils for the cathode and 3-9 mils for the polymer electrolyte/separator, with a total cell thickness of about 18-25 mils.
In the past, in such polymeric cells, the polymeric materials of the electrodes and separator have been of the same or very similar material. The cells were made with a lamination compression of the thin separator between the electrodes to enhance interface contact by means of a common material interface. The enhanced interfacial contact was deemed necessary to provide for sufficient ion transport capability through the separator to the electrodes, which ion transport had been minimized because of the low amounts of fluid electrolyte contained in polymer cells. Polymer cells contain all or nearly all of their electrolyte as absorbed within the electrodes and separator with little if any free liquid electrolyte. It was thus deemed necessary to provide a pressed laminate structure with all components having common polymeric components in order to provide the requisite interface for acceptable cell and battery performance. Accordingly, in polymer cells the separator was in the form of a thin membrane of the common polymeric material, having little structural integrity of its own. Accordingly the separator was required to be handled during cell and battery construction on a base substrate as a carrier until the separator was laminated with the electrodes, at which point the carrier substrate was removed.
However, despite the integration of anode, cathode and separator in polymer cells, as described, there were often fluctuations in cell performance in terms of cell rate, capacity and high temperature performance. This was in addition to the complications in manufacturing engendered by construction of such cells with component carrier substrates and the sensitivity of the material to mechanical handling.
By way of specific example, in a typical polymeric cell, the cell anode, cathode and separator are each comprised of a combination of a poly(vinylidene fluoride) (PVdF) copolymer matrix and a compatible organic plasticizer which maintains a homogeneous composition in the form of a film. In commercial embodiments, the separator copolymer composition comprises from about 75 to about 92% by weight of the poly(vinylidene fluoride) and about 8 to about 25% by weight hexafluoropropylene (HFP), (both commercially available from Elf AtoChem North America as Kynar FLEX.TM.), and an organic plasticizer. The copolymer composition is also used as binder material in the manufacture of the respective electrodes to insure a compatible interface with the separator.
The most common organic plasticizer materials are the higher-boiling point plasticizers including dibutyl phthalate, dimethyl phthalate, diethyl phthalate and tris butoxyethyl phosphate. In addition, inorganic fillers such as fumed alumina and silanized fumed silica were often added to enhance the physical strength and melt viscosity of the separator membrane, and to increase the electrolyte solution absorption level.
Because of the requisite thin film nature of the separators they were commonly made by procedures involving casting or forming films in addition to the described use of substrate carried membranes of polymer compositions.
In a typical lithium ion polymer cell the cathode is generally made of the polymer used in the anode and separator, with an amount of lithium manganese oxide added to the mix in place of the graphite (used in the anode) as a host intercalation material for the lithium ions.
The current collectors which are assembled to be in intimate electrical contact with the cathode and the anode are usually made from aluminum and copper, respectively, and of foil or grid-like configuration.
To facilitate ionic conductivity and transport, the anode, cathode and separator (usually as a unit, after lamination and compresion) are made porous by solvent extraction of the plasticizer material such as DBP, which, after the extraction, leaves matrices or pores in the electrodes and separator. It is understood however, that this is merely an illustrative embodiment and that other methods of extraction are possible, as well as other methods for providing the requisite porosity. The porous electrodes and separator are dipped into the electrolyte, prior to cell packaging, in order to load the electrolyte into the cell. Free liquid electrolyte is minimized or eliminated thereby.
The electrolyte, which is added to the electrodes and separator is comprised of a solution of a soluble lithium salt in one or more organic solvents such as ethylene carbonate and dimethyl carbonate (EC-DMC). Other commonly utilized non-aqueous solvents include .gamma.-butyrolactone (.gamma.-BL), tetrahydrofuran (THF), 1,2-dimethoxyethane (1,2-DME), propylene carbonate (PC), diethyl carbonate (DEC), methyl ethyl carbonate (MEC), diethoxyethane (DEE), dioxolane (DOL) and methyl formate (MF).
Generally the soluble electrolyte is present in about 1 to 2 molar solutions and with preferred and common soluble electrolyte lithium salts being LiPF.sub.6, LiAsF.sub.6, LiBF.sub.4, LiClO.sub.4, LiCF.sub.3 SO.sub.3, LiN(CF.sub.3 SO.sub.2).sub.3, and LiN(C.sub.2 F.sub.5 SO.sub.2).sub.3, with LiPF.sub.6 being particularly preferred.
Electrodes, anodes and cathodes type, structure and method of construction of polymer type cells are known in the field (e.g., such as described in U.S. Pat. No. 5,296,318) with carbon anodes and spinel cathodes such as of LiMn.sub.2 O.sub.4 and electrolyte containing separators, as described.